Six commercial cotton [Gossypium hirsutum] cultivars ['Deltapine 20', 'Deltapine 50', 'Deltapine 90', 'Deltapine 5690', 'Coker 315', and 'Paymaster 145'] were subjected to a 10-day laboratory post-emergent seedling vigor test at 15, 20, 25, and 30 C in which 30 C was treated as the optimal [control] growth temperature. The vigor test is further described in "Cotton Cultivar Responses to Suboptimal Post-emergent Temperatures" by J.M. Bradow, Crop Sci. 31:1595-1599 [1991]. Temperature treatments were applied from Day 4 through Day 10 post-germination. Capacity for recovery from exposure to suboptimal temperatures was also determined by returning a second set of stressed seedlings to 30 C from Day 8 through Day 10. Seedling growth parameters measured were root and shoot lengths and root and shoot fresh weights. Significant genotypic differences were found in all growth parameters and in the continuous stress and recovery capacity studies. Deltapine 20 and 50 were most sensitive to suboptimal temperatures, and Deltapine 5690 and 90 were least affected. The correlation of seedling growth inhibition [relative genotypic vigor] with yield is discussed in Part II which follows.